The campaign is really epic, the production values, storyline, and variability of the missions really make it an awesome experience. You have all your typical mission archtypes but in every mission there is a unique twist that makes things interesting, different, and rather challenging.
While it is very similar to the original; strategy-wise it's way different. Most of the old units no longer exist, and many of the ones that do function completely different. In the old game there were certain strategies for each race that were just "best", in the new one they've balanced the units so well you can come up with a huge array of strategies that will all work, or all fail depending on what your opponent does.
The only thing I don't like is the abilities for people to zerg really hard. Unless your a really good player, and your opponent is a really good player, you never get the chance to really unleash the full arsenal of any of the races. Multiplayer-wise this game has a long learning curve unless your one of the RTS multiplayer pro's. On the brightside they have a ranking system that helps keep you playing against people on your same skill level.
I would also suggest playing the Campaign on Hard, it makes the game more interesting and a much longer experience.
Here's a good review: http://pc.ign.com/articles/110/1108642p1.html